Communicating system



Dec. 15, L, D' WHITELOCK ET AL COMMUNICATING SYSTEM M THEIR ATTORNEYDec. l5, 1936. L. D. wHn'ELocK ET Al. 2,064,639

COMMUNICATING SYSTEM Filed June 9, 1934 3 sheets-smeet 2 -m l BH THEMATTORNEY Dec. 15, 1936. D WH|TEL0CK ET AL 2,064,639

COMMUNICATING SYSTEM Filed June 9, 1934 3 Sheets-Sheet 5 @Andreu/JSopenfen @if THEIR A T7 ORNE Y Patented Dec. 15, 1936 UNITED STATESPATENT GFFICE COMMUNICATING SYSTEM vania Application June 9, 1934,Serial No. 729,806

49 Claims.

Our invention relates to communicating systems, and particularly tocommunicating systems adaptable to telephone communication on railwaytrains.

The present application is a continuation in part of our copendingapplication Serial No.v

658,932, filed February 28, 1933, insofar as the subject matter commonto the two is concerned.

We will describe several forms of apparatus embodying our invention, andwill then point out the novel features thereof in claims.

Communicating systems have been proposed for telephoning between awayside station and a railway train, or between two trains, or betweentwo spaced locations on the same train, by transmitting from onelocation to the o-ther a carrier current modulated by voice frequencies.The communicating circuit preferably includes'the trailic rails in themanner disclosed and claimed'in the United States application forLetters Patent, 'Serial No. 450,135, filed May 6, 1930, by` L. O.Grondahl, for Electric train signaling system. In systems of the typehere involved the receiving apparatus at cach location is normallyactive and in circuit in order that either location can call the otherat any time. The transmitting apparatus is normally inactive and isenergized and in circuit only during the transmission of a current. Thereceiving apparatus preferably includes a loud-speaker and since 'thereceiving apparatus is normally in circuit, small earth currents andstray magnetic elds picked up during thenon-tiansmitting period of thesystem produce a noise in the loudspeaker that sometimes becomes veryannoying, especially since a high gain amplifier is employed.Furthermore, if two trains are close together, each train crew hears theconversation of the other and this condition often results in more orless confusion.

A feature of our invention is the provision of new and novel apparatusfor communicating systems of the type here referred to, wherewith theequipment at each location has a normal, a calling and a communicatingcondition any one of which may be readily established by the operator atthat location. Another feature of our invention is the provision of newand novel apparatus selectively responsive to a calling current having adistinctive characteristic, wherewith the receiving apparatus will notrespond unless a call intended for that particular location has firstbeen received. A further feature of our invention is the provision ofsimple and eiective means for reliably producing a calling currenthaving a distinctive characteristic to which the receiving apparatus atthe other location will readily respond. Still another feature of ourinvention is the provision of apparatus by means of which thetransmitting of a calling current is semi-automatic; that is to say, ifthe transmission of the calling current has been started by an operator,it will be automatically maintained for a predetermined interval,regardless of what action the operator takes in the meantime, theinterval of transmission being of suflicient duration to insure fullresponse of the receiving equipment at the location being called. Ourinvention also includes the provision of apparatus for systems of thetype involved wherewith sources of current of different voltages aremade available as such voltages are needed, together with apparatuswhich permits the supplying of current to the communicating circuit andthe picking up current therefrom through dilerent electrical coupling asdesired. Other features of our invention will appear as the specicationprogresses.

In the accompanying drawings, Fig. 1 is a diagrammatic view of one formof apparatus embodying our invention, wherein a sharply tunedcircuitnetwork is normally effective to receive a selecting call, and theloud-speaker is disconnected, or is in a condition of low sensitivity.The sending of the calling current is made semiautomatic, and oncestarted it will persist for a given period regardless of what action theoperator takes in the meantime. An inductor is provided for bothsupplying current to and picking up current from the trafllc rails theinductor being switched to the receiving apparatus or to thetransmitting apparatus as needed. Figs. 2 and 3 are each diagrammaticviews of different forms of the transmitting apparatus for generating acalling current of a predetermined frequency, both of which forms ofapparatus embody our invention. Fig. 4 is a diagrammatic view of asecond form of receiving apparatus embodying our invention, andwherewith the loud-speaker is normally'in a condition of low sensitivityby having a voice coil only in circuit; and is switched to a conditionof full sensitivity in response to the calling current by the energizingof its eld coil also. In this form of the receiving apparatus thecalling current is given additional amplification. Fig. 5 is adiagrammatic view of a. modified form of the apparatus ',of Fig. l whichalso embodies our invention. In

this modified form sources of current of different yoltages are providedand rendered available as needed. Current is supplied to the trame railsby a. circuit directly coupled with the rails and is picked up by aninductively coupled circuit which is normally sharply tuned to resonanceat the frequency of the calling current and more broadly tuned fortelephoning. In each of the several views, like reference charactersdesignate similar parts.

Referring to Fig. 1, the reference characters I2 and I3 designateinductor coils mounted on a railway train in inductive relation with thetraiiic rails II and I I", respectively. These inductor coils I2 and I3are normally connected with a iilter FI of the receiving apparatus to bedescribed later; and are adapted to be at times connected with atransmitter AT of the transmitting apparatus also to be described later.To provide two-Way communication between two spaced locations bothlocations would be equipped with both the receiving and the transmittingapparatus of Fig. 1. To clarify the following description, we shallassume that telephone communication is to be provided between thelocomotive and the Caboose of a freight train, and, consequently, thecomplete apparatus of Fig. 1 would be installed both on the locomotiveand in the caboose. Although a specific adaptation of our invention ishere disclosed, it will be understood that we do not wish to limitourselves to telephone systems for railway trains, but our invention isequally useful to other1 telephone systems of the type here involved.

In telephone systems for railway trains it has been proposed to attachto the microphone, indicated in Fig. 1 by the reference character M,

an operating handle I4 which is adapted to be manually operated to anyone of three positions indicated by dotted lines, and which positions weshall designate as normal, communicating, and calling, respectively. Aswill later appear, the normal position of the handle I4 places thetransmitting apparatus in an inactive condition, and connects thereceiving apparatus to the inductor coils I2 and I3. The communicatingposition of handle I4 places the transmitting apparatus in an activecondition, and the equipment is rendered ready to either receive ortransmit a telephone message. The calling position of handle I4 rendersthe transmitting apparatus active, and places the equipment in acondition for transmitting a calling current.

As shown schematically in Fig. 1, three circuit controlling contacts I5,I6 and I1 are operatively connected with the handle I4 and each of thesecontacts I5, I6 and I1 is moved to three respective positions identifiedin the drawings by the numerals I, 2 and 3, in response to the normal,communicating and calling positions of the handle I4. Mounted on thehandle I4 is a common spring return push button I8, which, as shownschematically, is operatively connected with a. circuit controllingcontact I9. Contact I9 is normally open and is forced down intoengagement with a contact 20 by the depressing of the push button I8.The function of these several circuit controlling contacts associatedwith the microphone handle I4 will appear as the specication progresses.

It was stated above that the inductor coils I2 and I3 are normallyconnected with the receiving apparatus, and in Fig. 1 the circuit forconnecting coils I2 and I3 to the receiving apparatusv can be tracedfrom the right-hand terminal of coil I3 along wire 22, lter FI, wire 23,back contact 2I of a directional relay DR to be referred to later, andwire 24 to the left-hand terminal of the coil I2, the two coils I2 andI3 being connected in series. 'Ihe back contact 2I and a front contact2|n of the directional relay DR are located in the drawings remote fromthe winding of that relay in order not to complicate the drawings. Itwill be understood that back contact 2| is closed when the relay isdeenergized and front contact 2| is closed when the relay is energized.Energy picked up from the traillc rails by the coils I2 and I3 andpassed by the lter FI is applied to the input of an amplifier AM overthe wires 25 and 26, and after amplification the energy is applied to ademodulator DI the output of which includes the primary winding 21 of atransformer TI. The filter FI is preferably a band-pass filter soproportioned and adjusted as to pass a band of frequencies having awidth substantially equal to the voice frequency range used; that is, itpasses a given carrier modulated with voice frequencies, or passes theside band frequencies of a carrier modulated with voice frequencies.Amplifier AM is any one of several types adapted to amplify a modulatedcarrier current, and the demodulator DI is any one of several typesadapted to demodulate a carrier modulated by voice frequencies and causeto appear in its output corresponding audiofrequencies only. The lterFI, amplier AM and demodulator DI may each be any one of severalwell-known types for such devices and are shown in Fig. 1 conventionallyonly in order to simplify the drawings as much as possible.

The secondary winding 28 of transformer TI is normally connected to acell selecting circuit and at other times is connected with an operatingcircuit. This call selecting circuit extends form the upper terminal ofthe secondary winding 28 through wire 29, tuned circuit network 9, frontcontact 3D of a relay R2 to be described later, and wire 3| to the lowerterminal of the secondary winding 28. The tuned circuit network 9includes a reactor 32 and a condenser 33 in series, a portion of reactor32 being connected to the input terminals of a rectifier 34, the outputterminals of which are connected with the winding of a relay RI. Theparts of this tuned circuit network are so proportioned and adjustedthat it responds only to a definite frequency within the voice frequencyrange, say, for example, 700 cycles per second. It will be understood,of course, that other frequencies may be selected should it seemdesirable to do so. 1t follows, that when a carrier current modulatedwith the frequency of 700 cycles per second is picked up from thetraflic rails, the relay RI is energized. At times the secondary winding28 is connected to an operating circuit including a loudspeaker`LS, andwhich can be traced from the upper terminal of secondary winding 28along wire 35, winding of loud-speaker LS, wire 36, back contact 38 ofthe relay R2 and wire 3I to the lower terminal of secondary winding 28.It follows that at such time as relay R2 is deenergizedin a manner to bepointed out later, the output of the demodulator DI is supplied throughthe transformer TI directly to the operating circuit for theloud-speaker LS; and at such time as relay R2 is energized, theloud-speaker is en- 31 maybe connected from the transformer TI and isinactive. As shown by dotted lines, a resistor 31 may be connected inshunt with the back contact 38 whereby the loud-speaker LS is normallyin a condition of low sensitivity and is then switched to a condition of'full sensitivity by deenergizing the relay R2 to close the back contact38 and form a shunt around the resistor 3l. It is clear from theforegoing, that energy picked up under the normal condition is appliedto the call selecting circuit and is ineffective to influence thatcircuit unless it is energy of the definite frequency to which the tunedcircuit network 9 is adjusted. Deenergizing the relay R2 disconnects thecall selecting circuit by opening the front contact 30, and connects theloudspeaker byclosing the back contact 38. 1n the event the resistor 31is provided,v the loud-speaker LS is normally in a condition of lowsensitivity and responds only feebly, and-is switched to a condition offull sensitivity when the relay R2 is deenergized, and then responds atfull volume. The relay R2, which controls the call selecting circuit andthe operating circuit for the loudspeaker LS, is normally energized overa stick circuit extending from the B terminal of a convenient source ofcurrent s uch as a battery not shown, through contact I'I in its No. 1position, wire 39, back contact i0 of relay RI, its own front contact4I, winding' of relay R2 and a manually operated contact 42. to the'opposite terminal C of the current source. Thus a calling current of thepredetermined frequency when picked up from the traflic rails andapplied to the call selecting circuit energizes the relay RI and thatrelay on picking up opens the back contact IIB in the stick circuit forrelay R2 to release that relay. A suitable amount of slow release actionis introduced in the operation of relay R2 by means of a condenser 3 anda resistor M, the two being connected in series across the winding ofthe relay R2. It is clear, therefore, that a short time after relay RIis picked up in response to the calling current, the relay R2 isreleased, disconnecting the call selecting circuit from the receivingapparatus and connecting the operating circuit of the loud-speakerthereto.

Operating the microphone handle I4 to the calling position to transmit acalling current in a manner to later appear will move the contact Il toits No. 3 position and relay R2 will be retained energized by currentfrom the B terminal of the current source over contact II in its No. 3position, wire d5, winding of relay R2 and manually operated contact t2to the terminal C of the current source. The function of this circuitjust traced will be pointed out later. Relay R2 when once deenergizedcan be reenergized by placing the handle It in its normal position tobring contact I l to itsNo. l position and then depressing a manuallyoperated contact 46. Cura l rent-will now be supplied from the Bterminal of the current source over contact I 1, wire 39, contact 46,winding of relay R2 and contact d2 to the C terminal of the currentsource. Once energized by this pickup -circuit just traced, the relay R2will be retained energized by its stick circuit described above. 4

It is clear from the foregoing description of the receiving apparatus ofFigul that any stray earth current that may be picked up by the inductorcoils I2 and I3 during the normal condition of the apparatus will not'be ,effective to create a noise in the loud-speaker. Furthermore,

energize the tuned circuit networkv with the result that theloud-speaker` will be shortly switched to full sensitivity ready torespond to any telephone message that may be delivered to thatl1ocation. At the termination of a telephone conversation the operator'can switch the loudspeaker back to its disconnected condition byoperating the contact 46 to reenergize therelay R2.

We shall n ow describe the operationA of the transmitting apparatus ofFig. 1. As stated hereinbefore, the operator will move the handle I4 tothe communicating position when he de-` sires to carry on a telephoneconversation, and he will move the handle I4v to the calling positionwhen he desires to transmit a calling cur.

manner in response to the energizing of that re-A lay. The transmittingrelay TR is made slightly slow releasing in character to prevent therelay from releasing during the operation of the contact i6 between itsNo. 2 and No. 3 positions. The closing of the front contact 5 is adaptedto render the transmitter AT active in a manner to shortly appear. I Iis placedat the communicating position the depressing of the push buttonI B to close the contact Iii-20 supplies current from the B terminal ofthe current source over contact Iii-2li, contact I5 in its No. 2position, wires |52 and $5, winding of the directional relay DR and tothe C terminal of the current source, and the directional relay DR isenergized. The inductor coils I2 and I3 are normally connected with theiilter FI of the receiving apparatus over the back contact 2I-of thedirectional relay DR as described above. Energizing the directionalrelay DR in the manner just pointed out transfers the coils I2 and I3 tothe transmitting apparatus, the

circuit including the front contact Zia of the directional relay DR. Tobe explicit, this last circuit can'pbe traced from the right-handterminal of the coil I3 over Wires 22 and 4l, secondary windingv MI of atransformer T6 of the transmitter AT, wire 4B, front contact 2lav andwire 2li to the left-hand terminal of coil I2. It follows that at suchtime as the handle IB is set at the communicating position, the operatorcan connect the coils i2 and I3to either the receiving apparatus or tothe transmitting apparatus by operating the pushbutton I8 to energize ordeenergize the directional relay DR. The directional relay DR alsocontrols a front contact DR adapted to govern the operation of thetransmitter AT as will later appear. Front contact DRd is shown in Fig;l remote from the Winding of that' relay for clarity, and it will beunderstood that this front contact is closed in the usual manner whenrelay DR is energized;

The microphone M is provided with a circuit easily traced and whichincludes a battery 49, back contact 50 of a calling relay CR to bereferred to later and the primary winding 5I of a transformer T2.Consequently, variations produced by speaking into the microphone Mcause variations of the current flowing in the At such time as thehandle in turn induce corresponding ,frequencies in the primary winding5I of transformer T2 which secondary winding 52 of transformer T2. Thevoltages induced in the secondary winding 52 are applied to the controlgrid 53 of an amplifying tube 54, a suitable biasing battery 55 beinginserted in the connection between the secondary quencies induced in thesecondary winding 52 of transformer T2 are repeated in the primarywinding 53 of transformer T3 greatlyincreased in amplitude, and in turninduce an electromotive force of corresponding frequencies in thesecondary winding 6| of that transformer.

'I'he transmitting apparatus includes a generator G of carrier currentof a frequency of, say, for example, 7,000 cycles per second, althoughit will be understood that other frequencies may be selected if founddesirable. 'I'he carrier current Agenerated by the generator G ismodulated with the frequencies induced in the secondary 6| oftransformer T3 in the usual manner. and the modulated carrier output ofgenerator G is delivered to the input of the amplifier-transmitter ATover wires 52 and 53. The generator G shown conventionally in Fig. 1 maybe any one of several w`ell known types of generators of carrier currentsuch for example as the vacuum tube type.

The amplifier-transmitter AT is preferably of the well-known push-pullamplifier type and includes two electron tubes |42 and |43. Thefilaments |44 of tubes |42 and |43 are heated by current supplied from abattery |45 over a simf ple circuit easily traced and which includes thefront contact TR5 of the transmitting relay TR. The grid |45 of tube |42is connected to one output terminal of the generator G over wire 62, andthe grid |41 of tube |43 is connected to the opposite output terminal ofthe generator G over wire 63. 'Ihe plate |43 of tube |42 is connected tothe top terminal of the upper half of the primary winding |43 of thetransformer T6, and the plate of the tube |43 is connected to the lowterminal of the lower half of the primary winding' |49. The mid point ofthe primary winding |49 is connected to the positive terminal of arelatively high voltage current source, not shown, over the frontcontact DR4 of the directional relay DR. The negative terminal of thiscurrent source is connected to the filaments of tubes |42 and |43 over awire I5I. It follows that the transmitter AT is normally deenergized andbecomes energized when- `ever either the communicating or callingcondition is established and the relays DR and 'I'R are picked up. Thefilament circuit for tubes |42 and |43 being completed at the frontcontact TR5 of the relay TR, and the plate circuits for these tubesbeing completed at the front contact DR4 of the relay DR. Thus thefrequencies applied to the grids of the tubes I 42 and |43 from thegenerator G are repeated in the plate circuits of these tubes greatlyincreased in amplitude, and .corresponding frequencies'are induced .inthe secondary winding I4I of the transformer T5 amplified to arelatively high energy level.

accesso The energy induced in the secondary winding I4| is supplied tothe inductor coils I2 and I3 whenever the directional relay DR isenergized. It is clear. therefore, that with the handle I4 set at thecommunicating lposition and push button I5 is depressed, variationsproduced by speaking into the microphone M are converted intocorresponding frequencies which are first amplified and then used tomodulate a carrier current. 'Ihe modulated carrier current is furtherampliiied to a relatively high energy level and then applied to thecommunicating circuit.

A calling current is produced in the following manner. Moving thehandleA |4 to the calling position supplies current from thev batteryterminal B over contact I5 in its No. 3 position, wire 64, winding ofthe calling relay CR, wire 55,- Winding of the directional relay DR andto the C terminal of the battery; and the relays CR and DR are energizedin series. Relay CR on picking up opens the microphone circuit at. theback contact 50 and closes a front contact 66 to couple the platecircuit of tube 54 to its grid circuit through a coupling condenser C2.Opening the back contact 50 in the microphone circuit causes theimpedance between the grid 53 and the iilament 61 to greatly increase,since,

with the shunting effect of the microphone circuit removed, theimpedance between the grid 53 and the filament 61 i's limited only tothe transformer losses of transformer T2 and occurs at a frequencydetermined by the resonant frequency of theV inductance of thetransformer and its distributed capacity, plus the capacity of the tube54 and other stray capacities in the circuit.

This increase in the impedance of the grid cir' cuit would normally besufficient to cause the tube 54 to oscillate, but, in order to stabilizethe oscillations the coupling condenser C2 is connected with the gridand the plate by means of the front contact 55 of relay CR. 'I'hecondenser C2 augments the transfer of the energy of the plate circuit tothe grid of the tube and insures that there is sufficient feed-back toovercome the dampened effect of any losses in the grid circuit. Thecapacity of condenser C2 and the parts of the circuit associated withthe tube 54 are so proportioned and adjusted that a single frequency ofsay 700 cycles per. second is generated when the relay CR is picked up.The

carriercurrent generated by the generator G' is now modulated by thesingle frequency of the calling current produced by the tube 54 andl itsrelated circuits. Hence, the calling current is a carrier having afrequency of 7000 modulated with the frequency of 700 cycles, and sincethe relay TR is also picked up and transmitter AT rendered active thiscalling current is delivered to the trafic rails amplified to arelatively high energy level by the` transmitter AT.

The semi-automatic transmission of the calling current takes place asfollows; Let us assume that the microphone handle I4 is in its normalposition and that the operator desires to call the other location. Theoperator will first throw this handle I4 to the calling position andcause the relayshTR, CR and DR to be energized. Relay TR on- -picking uprenders the transmitter AT active, relay-CR on picking up causes tube 54to generate a calling frequency of 700. cycles, and as the coils I2 andi3 are now connected to the transmitter AT by the front contact 2|* ofthe relay DR, the traiilc rails are supplied with the calling current.Ihe

No. 3 position of Contact I'I supplies current directly to the relay R2and that relay is retained energized and -relay DR being now picked up.current flows from the battery terminal B over front contact 68 of relayR2, front contact 69 of relay DR. winding of the calling relay CR, wire65, winding of the relay DR and to the C terminal of the battery, anindicating lamp 10 being connected in parallel across the windings ofrelays CR and DR. It follows that whether the microphone'handle I4 iskept in the calling position or restored to one of its other positions,the two relays CR and DR will both remain energized until the relay R2is released. Moving the handle I4 away from vthe calling position opensthe circuit to the relay R2 and that relay is deenergized; however,relay DR on picking up closes at its front contact I'II a shunt patharound the resistor 44 and the condenser 43 is therefore directly acrossthe winding of relay R2, causing the release period of that relay to'beconsiderably longer than when the resistor 44 is in series withcondenser 43. Hence, having once started the transmission of the callingcurrent, the operator is free to move his handle I4 from the callingposition to the communicating position and make ready to startconversation.

At the other location, the receipt of the calling current will be pickedup from the traffic rails, filtered, amplified, demodulatecl and currentof a frequency of 700 cycles per second will be supplied to the callselecting circuit. The tuned circuit network 9 will respond to thisfrequency of '700 cycles per second and the relay Rl will be energizedto open the stick circuit of relay R2 and that relay will release at theend of the slow release period as determined by the associated condenser43 and the resistor 44 in series. Hence, in a short time the relay R2 atthe called location releases to disconnect the call selecting circuitand to connect the operating circuit of the loud-speaker with the resultthat the loudspeaker will now sound a note corresponding to thefrequency of 700 cycles. It will be noted that the release of the relayR2 at the location which is being called is determined by the associatedcondenser 43 and resistor 44 in series, and will be materially shorterthan the release period of the relay 42 at the calling location which isdetermined by the associated condenser J43 alone. Consequently, thecalling current will be transmitted for a period sufficiently long tocause the receiving apparatus at the location which is being called totransfer its loudspeaker to a condition of full sensitivity and to sounda note calling the operators attention that communication is desired,notwithstanding the calling operator is free to at once place hismicrophone handle I4 in the communication position immediately afterinitiating the calling current. At the calling location the release ofits relay R2 terminates the transmitting of the calling current byopening the circuit to the CR relay, which in turn upon releasingdisconnects the coupling condenser C2 and reestablishes the microphonecircuit. The indication lamp I will be displayed during the time thecalling current is being transmitted, and will be extinguished when theR2 relay releases, indicating to the operator at the calling locationthat the impulse of calling current has been terminated and that hisapparatus is in a condition to start conversation. At the calledlocation the '700 cycles of the calling current will cause the loudspeaker to sound a note indicating to the operator at that location thata telephone mes sage is about to be delivered as stated above. Theoperator at the called location will respond by moving his microphonehandle I4 to its communicating position and from this point onconversation can be had between the two locations by the operatorsmanipulating their respective push buttons I8. At the end of theconversation each operator can restore his loud speaker and itsassociated call selecting circuit to their respective normal conditionby depressing the switch 46 to reenergize the associated R2 relay. Inthe case it is desired to establish the loudspeaker in a condition offull sensitivity and still have the microphone handle I4 in the normalposition, the operator can do so by either depressing the switch 42 long`enough to bring about the release of the R2 relay or by moving handleI4 to theNo. 2 position just long enough to release relay R2, and thenrestoring handle I4 back to the No, 1 position. At stated above, to cutthe loud-speaker down to its low sensitivity at any time it is necessaryonly for the operator to depress the switchy 46 to energize the R2 relayafter which it will be retained energized over its stick circuit. f

The calling current having a modulation frequency in the voice frequencyrange about where maximum energy is transmitted, namely, 700 cycles persecond, the calling current has been found to be effective to stronglyactuate the receiving apparatus at the remote location even undertransmitting conditions so poor or at distances so great that thetelephone current is so attenuated a telephone message is only feeblyreceived. That is to say, a note of the loudspeaker corresponding to thefrequency of the calling current may still be clear and distinct underconditions where the telephone message reproduced by the loud-speakermay be unintelligible At such times the calling apparatus becomesvaluable as a means for transmitting a code message from one location tothe other. Assuming .the operator at one location desires to communicatewith the operator at the other location by a code message, the procedureis as follows: It will be understood, of course, that diierent codes ofshort and long impulses corresponding to diierent code messages areprearranged and are familiar to the operator at each location. Movingthe handle I4 to the calling position causes the transmitting apparatusat that location to become active to supply calling current to thetraffic rails in the manner described hereinbefore. The impulse ofcalling current is picked up at the remote location and the loud-speakerautomatically connected to the output of the transformer TI and acorresponding note sounded in the manner described previously. 'Ihesending operator terminates an impulse of calling current by moving thehandle I4 back to the communicating position after which the callingcurrent ceases as soon as the relay R2 releases. To start a secondimpulse of calling current the operator moves the handle I4 back to thecalling position to again connect the coils I2 and I3 to thetransmitting apparatus and to cause calling current to again begenerated. It follows that the sending operator by operating his handleI4 between the No, 2 and No. 3 positions in accordance with long andshort intervals causes impulses of calling current corresponding to aprearranged code to be delivered to the traic rails, and the desiredcode message is reproduced by the loudspeaker at the other location bysounding corresponding long and short; notes. It is to be noted that theslowrelease feature provided for maintaining the impulse of callingcurrent for a period equal to the release period of the relay R2 afterthe handle I4 is moved from the No. 3 position does not hamper thesending of the code message. The sending operator by watching theindicating light 18 can tell each time an impulse has been terminatedand,v hence knows when he is free to start the next impulse. It is cleartherefore that by operating the handle I4 in accordance with anyprearranged code, a code message' can be readily communicated from onelocation .to the other-by the apparatus of Fig. 1. Although it has beenassumed above that code messages are transmitted under conditions inwhich telephone communication is not satisfactory, it is apparent that acode message can be transmitted in a manner just pointed out at anytime. As a matter of fact the transmitting of a code message may attimes be desirable to communicate an order quickly without the necessityof waiting for the receiving operator to indicate back to the sendingoperator that he is ready to receive a telephone message.

It has been proposed for carrier telephone systems to transmit but asingle side band of the carrier modulated with voice frequencies,suppressing the carrier and the other' side band. It has further beenproposed to make use of a pentode type electron tube in the generatingof the carrier current, applying to one grid the voice frequencies withwhich the carrier is to be modulated. Fig. 2 discloses a form of thetransmitting apparatus of a side band telephone system embodying ourinvention, wherewith the desired single frequency for the callingcurrent is obtained by shifting the frequency of the unmodulated'carrier by an amount corresponding to the frequency desired. Forexample, with a carrier of 7000 cycles per second and a desired callingfrequency of '700 cycles per second, the frequency of the carrier isshifted '100 cycles up or down depending upon which of the side bands isbeing used for the transmission of speech. Referring to Fig. 2, theinductor coils I2 and I3 are adapted to be connected to either the lterFI of the receiving apparatus, or to 'be connected with the transmitterAT of the transmitting apparatus by virtue of the directional relay DRand the push button I8 associated with the microphone handle I4 the sameas described in Fig. l. The output of a modulator-oscillator indicatedas a whole by the reference character MO is applied over wires 1I and 12to a bandpass filter BPF adapted to substantially suppress the carrierand one side band passing only the remaining side band, preferably theupperv side band, to the input of an amplifier AMI. Amplifier AMIamplies the upper side band and delivers the energy to the input of thetransmitter AT which in turn supplies the upper side band frequencies tothe coils I2 and I3 at a relatively high energy level. The filter BPFmay be any one of several types of band-pass filters and in thisinstance is so proportioned and adjusted as to pass the upper side bandfrequencies having the width of the voice frequency range, of a carrierof '7000 cycles. The amplifier AMI may be of any of the several standardtypes commonly used for amplifying a. modulated carrier current and ishere shown conventionally only in order to not unduly complicate thedrawings. The transmitter AT shown conventionally in Fig. 2 ispreferably similar to the transmitter AT of Fig. 1 and is governed bythe relays DR and 'I'R the same as in Fig. 1.

'I'he modulator-oscillator MO includes a pentode 13 having a filament14, a plate 15 and two control grids 16 and 11, the filament 14 beingheated by a battery 18. 'I'he plate circuit includes the usual inductor19 and a plate battery 88 as will be readily understood by an inspectionof Fig. 2. One terminal of an oscillating circuit 88 comprising aninductor v8| and a condenser 82 is connected with the plate circuitthrough a blocking condenser 83, and; its other terminal is connectedwith the grid 16, a mid tap of inductor 8I being connected to thefilament 14 through a suitable biasing battery 84. The parts of thisoscillating circuit are so proportioned and adjusted that a carrierfrequency of 7000 cycles per second is generated and applied to thewires 1I and 12 leading to the bandpass filter BPF. 'I'he microphonecircuit includes the back contact 50 of the calling relay CR and theprimary winding 5I of the transformer T2 the same as in Fig. 1. Oneterminal of the secondary winding 52 of transformer T2 is connected tothe second control grid 11 of tube 13, and its other terminal isconnected to the filament 14 through a battery 85. It is clear thatvoice frequencies developed in the microphone M induce correspondingfrequencies in the secondary 52 which are applied to the grid 11 withthe result that the carrier frequency is modulated with the voicefrequencies.

Operating the microphone handle I4 of Fig. 2 to the calling position tobring contact I5 to its No. 3 position supplies current to the windings35 of relays CR and DR in series to energize those relays. Relay DR onpicking up completes the circuit at its front contact 69 for retainingthe two relays CR and DR energized until such time as the associatedrelay R2 is released irrespective of the position of the handle I4 thesame as described in Fig. l. With the calling relay CR picked up theopening of the back contact 50 renders the microphone circuitineffective, and the closing of the front contact 81 connects aninductor 86 in parallel with a portion of the inductor 8i of theoscillating circuit 88. Connecting inductor 86 in parallel with inductor8I varies the inductance of the oscillating circuit 88 to increase thefrequency generated. The inductor 86 is so selected that the carrierfrequency is shifted 700 cycles making the frequency now delivered tothe band-pass filter BPF '7700 cycles per second. This single frequencyof the calling current falls within the width of the band to which lterBPF is adjusted, and is passed to the amplifier AMI and in turn to thetransmitter AT. It follows that as long as the calling relay CR and thedirectional relay DR are picked up a calling current of 7700 cycles persecond is supplied to the traffic rails. This calling current whenpicked up at the other location will influence the receiving apparatusat that location in the manner described in detail for Fig. 1. If it isdesired to shift the frequency of the carrier down, the inductor 86.would be replaced by a' suitable condenser. It is to be noted that bychanging the frequency in the manner disclosed in Fig. 2, it is possibleto avoid having relay contacts in the oscillating circuit during suchperiods as conversation is to be transmitted. Otherwise, a change in theresistance of a relay contact may alter the frequency of the oscillatingcircuit enough to cause poor ltransmission of a telephone message. v

.It has beenv'proposedl for single side band transand by changing theconnection of the'2-tube` balanced modulator so that the carrierfrequency instead of being balanced out adds its effect in the outputcircuit. Referring to Fig. 3, the inductor coils I2 and I3 are adaptedtoy be connected to the filter FI of the receiving apparatus or to thetransmitter AT of the transmitting apparatus by virtue of thedirectional relay DR and the push button I8 associated with themicrophone handle N the same as described hereinbefore. The microphonecircuit includes the primary winding 5I of the transformer T2 with theresult that variations produced in the microphone induce correspondingvoice frequencies in the secondary 52 of transformer T2 the same as inFigs. 1 and 2. "In this instance the voice frequencies induced in thesecondary winding 52 are applied to the grid 89 of an amplifying tube90, a suitable biasing battery 9| being inserted in the connectionbetween the secondary winding 52 and the lament 92 of tube 90. The plate|90 of tube 90 is connected with a circuit that includes the twoportions 93 and 94 of the primary winding of a modulating transformerT4, the secondary windings 95 and 96 of which supply the amplified voicefrequencies to the grids 91 and 90 of the two tubes 99 and |00 of abalanced modulator-oscillator designated as a whole by the referencecharacter BMO. The secondary windings 95 and 96 of transformer T4 formone continuous winding` arranged in two equal parts with a mid tap to beshortly referred to.

The balanced modulator-oscillator BMO comprises two electron tubes 99and |00 of the standard 3-element type and a conventional oscillatingcircuit |06. The grid circuits of tubes 99 and |00 and also their platecircuits are arrangedV in the conventional push-pull manner; and theoscillating circuit |06 is adapted to have its generated carrierfrequency shifted as will shortly appear. The plates |03 and |04 oftubes 99 and |00, respectively, are connected to the outside terminalsof the primary windings |I and |02 of an output transformer T5, theoutside terminal of primary winding |02 being connected directly to theplate |04 of tube |00 and the outside'terminal of the primary winding|0I being connected with the plate |03 of tube 99 over a back contact|05 of the calling relay CR. The common center terminal of windings |0|and |02 is connected with the positive terminalof a plate battery IIIthrough an inductor I I2, the opposite terminal of battery III beingconnected to the filaments of these tubes. The oscillating circuit I 06comprises a portion of an inductor 01 and two condensers |08 and |09 inparallel, the connection for the condenser |09 including a back contact||0 of the calling relay CR. One terminal of this oscillating circuit isconnected through a blocking condenser H3 to the plate circuits fortubes 99 and |00 at the mld point of the primary windings |0| and |02;and its other terminal is connected through the lower portion' o'finductor |01 and a suitable biasing element H4 to the grid circuits atthe mid point of the secondary windings 95 and 96 of transformer AT4; amid tap ||5 of inductor |01 being connected to the filaments of vthe twotubes. It is clear that the tubes 99 and |00 are operated in parallelwith the grid terminal of the oscillating circuit |06 connected to thetwo grids of the Y tubes through the secondary windings of transformerT4, and the plate terminal ofthe oscillating circuit connected to thetwo plates of the tubes through the blocking condenser ||3 and theprimary windings of transformer T5. 'The result of this arrangement isthat there will be no carrier transmitted to the secondary winding I|6of the output transformer T5, since the primary windings |0| and |02 arearranged in the push-pull manner and the oscillations supplied to theplate circuits are balanced out. The addition and subtraction of thevoltages corresponding to ,the voice frequencies with the voltages ofthe `carrier frequencies when applied to the two grids 91 and 98 causethe grids to be opposite `in polarity with the result that thevariations produced inthe plate circuits of these tubes corresponding tothe side bands are additive in effect in the transformer T5. The twoside band frequencies corresponding to the carrier modulated with voicefrequencies are vhence supplied by the transformer T5 to the band-passlter BPF which in this instance is so proportioned and adjusted as toselect one side band and suppress the other, preferably the upper sideband being passed. The upper side band is then applied to the input ofthe amplifier transmitter AT, and. hence is delivered to the inductorcoils I2 and |3 at relatively high energy level. An amplier similar tothe amplifier AMi of Fig. 2 may be inserted between the band-pass filterBPF and the transmitter AT should it seem desirable to do so.

To provide a calling current the carrier frequency generated by themodulator-oscillator BMO, instead of being balanced out in the secondarywindings of the output transformer T5 is made to add, and at the sametime its frequency is shifted by an amount corresponding to the desiredmodulation frequency, the change in the frequency being eitherup or downdepending upon-whether the upper or lower side band is the one beingutilized for conveying the telephone message. oscillating circuit |06 isso proportioned and adjusted as to generate a carrier frequency of 7000cycles per second, and that the Lipper side band is selected by theband-pass filter BPF; and that a modulation frequency of '700 cycles persecond is desired for calling. That is to say, the carrier frequency of7000 cycles is to be shifted to 7700 cycles for calling. Operating themicrophone handle I0 to the calling position supplies current to the tworelays CR and DR in series the same as in 1, and these relays then remain energized until such time as the associated relay R2 is released,notwithstanding the fact that the handle i4 may be moved to position No.2 in the meantime. Opening the back contact |05 and the closing of thefront contact i1 transfers the plate |03 of tube 99 from the upperterminal of primary winding |0I to the lower terminal of the primarywinding |02 to which the plate |04 of tube |00 is also connected. Theresult is that the plate circuits of tubes 99 and |00 instead of havinga push pull arrangement are now in parallel each including the primarywinding |02. Relayr CR on picking up also opens the back contact I|0 Weshall assume that the and' the condenser |09 is removed from theAoscillating circuit causing the frequency of this circuit to beincreased. The condenser |99` is so selected ,that when it is removedfrom the oscillating circuit, the frequency o f the carrier is increasedfrom 7000 cycles to 7700 cycles. It follows that as long as .the callingrelay CR isenergized, a carrier frequency of 7700 cycles is passed tothe output transformer T5 at full intensity of the two tubes operatingin para1lel,- instead of being balanced out by the push-pull arrangementin eiect when a telephone conversation is to be transmitted. Since thefrequency of the calling current, namely 7700 cycles per second,

lies within the voice frequency range of thel band-pass filter BPF, thecalling frequency is passed to the transmitter AT and hence is de--livered to the inductor coils I2 and I3. This calling current of thesingle frequency of '1700 modulationfrequency supplied by the secondarycycles per second when picked up at the other location and applied toits receiving apparatus will cause that apparatus to function in thesame manner as described for Fig. 1. is desired. to have the callingcurrent corresponding to a frequency of the lower side band, the

condenser |09 would be normally disconnected from the oscillatingcircuit v|00 and then connected into the circuit when the calling relayCR is energized.

The modiiied form of the receiving apparatus disclosed in Fig. 4provides an additional ampliflcation for the calling current. In thisform of apparatus embodying our invention the output transformer TI hasits primary winding 21 connected with the demodulator DI the same as inEig. 1. 'Iransformer TI, however, is provided with two secondarywindings |-|3 and |20, the secondary winding |I9 being connecteddlrcctly with a voice coil I I9 of the loud-speaker IS, and thesecondary winding |20 being connected with the control grid I2I of apentode and to the opposite terminal C of the current which includes asuitable biasing battery |23. The plate |24 o f tube |22-is connected toa battery |26 through a reactor |21 to form the usual plate circuit, andthe screen grid |25 is connected directly to the positive terminal ofthe' battery |26. Twocondensers |29 and |29 are connected in seriesbetween the plate and the filament of tube |22 and these condensersarepreferably made of substantially the same size and have a comparativelylarge capacity so that any high frequency current which may `flow in theplate circuit will be by-passed to the ills.- ment. A reactor |30 isconnected across lthe condenser |29-, and it will be noted that due tocondenser |23 acting as a blocking condenser, no unidirectional currentwill flow in reactor |30. A portion of reactor |30 is connected to thesharply tuned 'circuit network 9 which includes the 'condenser 33 and.reactor 32 .in series the vsame as described hereinbefore. A portion ofreactor 32 is connected to the input terminals of the rectier 34, theoutput terminals. of lwhich are connected to' the top winding of 'arelay R4. The alternating current corresponding to the |20 causescorresponding V variations in the plate circuit ofv -tube |22, andsimilar/variations of In case it ceiving apparatus is. modulated at apredetermined frequency, a portion ofthe output of the demodulator DI isappliedacross the grid and filament of vthe'tube |22 where it is`amplified '700 cycles per second, it is`possible to operate relay R4 bythe voice at thetransmitting location.

A-ne'ld 611 m 'of the vloud speaker Ls is adapted to be energized byeither one of tw`o circuits. The first circuit can be traced from theterminal B of the current source over coil terminal B through the lowerAor holding winding of relay R4,v its own front contact |35, wire |36,contact Il in its No. 1 position and `thence to the battery terminal C.Hence, relay R4 once picked up by current supplied4 over the selectingnetwork 9, will be retained energized by current supplied from the localsource until such time as the microphone handle I4 is moved out of itsnormal position. 'I'he energizing of relay R4 in the manner justdescribed causes the eld winding I3I of the loud-speaker to beexcitedand the loud-speaker operated in a condition of full sensitivity.The operator in moving the microphone handle I4 from its normallposition to the communicating position opens the circuit to the holdingwinding of relay R4 and that relay is released, but the circuit to thefield winding |3| is retained closed by the circuit that includes thecontact I'|.in its No. 2 position, and hence the loud-speaker lLS isretained in the condition of. full sensitivity during a telephoneconversation. -A rectifier |31 may be connected across the field windingI3I as shown -to suppress any spark or arc at the front contact |33 orat the'contact I1.

If the operator desires to place the loud-A speakerLS at fullsensitivity when no calling current has'been received, he can do so bydepressing a `spring return switch I 38 to energize the holding windingof relay R4 by a circuit easily traced and which includes the contact I1in its No. 1 position, Relay R4 once picked upY position. In case it isnot necessary to answer a received telephone message and the microphonehandle I4 is not moved outof its normal position, the loud-speaker LSisleft at full'sensitivity at the termination of the telephone messagebut it can berestored to the condition of low sensitivity by theoperator moving the handle |4 away from the normal position for a shortinterval of time. In Fig. 4, the field coil |3| being normallydeenergized, the resultant 75 reduction in sensitiveness of theloud-speaker will be ample to remove any noise due to static conditions,and yet a calling signal may still be heard faintly even if the callselecting cincuit fails, the voice coil ||9 being in circuit at alltimes.

In the form of our invention disclosed in Fig. 5, a power unit forsupplying the proper plate voltages to the various electrontubes isprovided, the arrangement being such that the power unit is renderedactive as needed and the generated voltages switched to the platecircuits of the sending and receiving tubes according to whether currentis to be transmitted or is to be received. In this form of ourinvention, current is supplied to the trafiic rails over a circuitdirectly coupled therewith and is picked up through the medium of areceiving coil tuned sharply for calling and the tuning of which isbroadened for telephoning either automatically or manually.

Referring to Fig. 5, the microphone M and the associated circuitcontrolling contacts and push button are shown as a unit designated bythe reference character MU, the circuit controlling contacts beingoperatively connected with the microphone handle and the push buttonmounted thereon the same as in Fig. l. The microphone circuit is easilytraced and includes the microphone M, the primary winding 5| of thetransformer T2, a reactor 253 and a current source as will readily beunderstood by an inspection of the drawings. It follows that anelectromotive force having frequencies corresponding to the voiceimpulses produced in the microphone M is induced in the secondarywinding 52 of the transformer T2.

The power unit designated by the reference character PU consists of twodynamotors or motor generators DMI and DM2 together with filter devicesto be referred to later. Energy is obtained fo'r energizing the motorelements 6 and 1 of the two dynamotors from any convenient source suchas a headlight generator not shown, the opposite terminals of which aredesignated by the reference characters B32 and N32. A common double-polecut-out switch |12 is preferably interposed between the source of powerand the power unit, and the B32 wire is preferably connected with aground electrode 240. With the cut-out switch |12 closed, current isconstantly supplied to the motor element 6 over a simple circuit whichincludes a choke coil |13. With the switch |12 closed and the microphonehandle |4 of the unit MU operated to either its communicating or callingposition and the contact |14 moved to its No. 2 or No. 3 position,current is supplied to the motor element 1 of the dynamotor DM2 by asimple circuit easily traced and which includes the contact |14 and achoke coil |15. It follows that as long as the cut-out switch |12remains closed, the dynamotor DMI is operated to develop a voltageacross its generator element 8, and when the microphone handle I4 is setat either its communicating or calling position, the dynamotor DM2 isalso operated to develop a voltage across its generator element I0. Tofix the ideas, we shall assume that the source of energy B32-N32 is of32 volts direct current, and that cach of the dynamotors develop 500volts direct current, the polarity of the brushes of the generatorelements 8 and I0 being as indicated by the plus and minus signs placedon the drawings.

In this form of the invention, the directional relay DR is made a partof the power unit, the

relay DR and the calling relay CR being controlled, however, in a mannersimilar to that of Fig. 1. It is thought unnecessary to repeat thedescription of the control of these relays except to point out thatunder the normal position of the handle I4 both relays are deenergized,under the calling position of the handle I4 both relays DR and CR areenergized over a circuit including the No. 3 position of the contacts I5and |14, respectively, While the communicating position of the handle I4permits the directional relay DR to be picked up or released bydepressing or releasing the push button I8, the circuit including thepush button depressed and the Contact |14 in its No. 2 position. Thedeenergized condition of the directional relay DR connects over its backcontact |16 the positive brush cf the generator element 8 with the BAterminal of the power unit, a choke coil |11 being interposed in theconnection; and connects over the back contact |18 the negative brush ofthe generator 8 with the terminal NB of thepower unit, this terminal NBbeing in turn connected with a ground electrode |19. The energizedcondition ci the directional relay DR connects. over its front contact|80 the positive brush of the generator 8 with the terminal BPA of thepower unit and also with the negative brush of the generator element I0,a choke coil I8| being interposed between the contact |80 and theterminal BPA. The negative brush of the generator 8 is now connectedover the front contact |82 with the terminal NP of the power unit. Thepositive brush of the generator element I8 is connected over a chokecoil |83 with a terminal BP of the power unit. Consequently, under thenormal position of the microphone handle I4 and with the directionalrelay DR deenergized, the dynamotor DMI only is operated and a ptentialdifference of 500 volts is established between the terminals BA and NB.With the handie I4 moved to either the communicating or callingposition, the second dynamotor DM2 is started up and if the directionalrelay DR is energized, as it will be in the calling position and as itmay be during the'communicating position by the depressing of the pushbutton I8, a difference of potential of 500 volts exists between theterminals BPA and NP and a diierence of potential of 1000 volts existsbetween the terminals BP and NP. To suppress sparking and to avoidpotential surges, condensers and leak resistors may be connected aroundthe contacts |18-I82 and |16--I80 of the relay DR as will be understoodby an inspection of Fig. 5. The several choke coils of the power unitare effective to filter out the ripples produced at the commutators ofthe dynamotors and of source B32- N32. The manner of utilizing thevoltage delivered by the power unit will appear as the description ofthe apparatus of Fig. progresses.

The transmitting apparatus of Fig. 5 includes, in addition to themicrophone M, a transmitter AT, a frequency translating unit designatedas a Whole by the reference character- TFT, and a circuit for supplyingthe outgoing current to the traiiic rails. The receiving apparatusincludes, in addition to the loud-speaker LS, a frequency translatingunit indicated as a whole by the reference character RFT, and areceiving coil RC for picking up current from the communicating channel.As stated hereinbefore, in practicing our invention, the receivingapparatus is made normally active ready to receive at any time anincoming current, while the transmitting Y,

apparatus is normally inactive and is rendered active only when it isdesired to send a current.

It will be understood, of course, that each loca- RFT have theirfilaments constantly heated and have a voltage applied to their plateelements with the result that whenever current is picked up by thereceiving coil and applied to the.-grids of the several tubes, theapparatus functions as intended. The electron tubes provided for thefrequency translating unit TFT of the transmitting apparatus preferablyhave their laments heated also under the normal condition in order thatthere may be no delay in the transmitting apparatus assuming a conditionfor transmitting a current. However, the source of plate voltage forthese sending tubes is normally disconnected from the plate elements andhence the transmitting apparatus is normally inactive. The power tubesof the transmitter AT of Fig. 5 normally have no current applied toeither the filaments or to the plates the same as in Fig. 1. To heat thefilaments of the tubes of the two frequency translating units, a circuitis provided for serially connecting the filaments of the several tubesacross the source B32-N32, this circuit being traced from the B32terminal over resistor |85, filaments of tubes |89, |88, |81, and |84 tothe terminal |90 of the unit RFT, and thence to the terminal |9| of theunit TFT as indicated by the arrows, (the connection between theseterminals being omitted for simplicity), filaments of the tubes |92 and|93, and wire |94 to the terminal N32 of the source of power. It will benoted that the laments of the power tubes |42 and |43 of the transmitterAT are heated by a circuit extending from the B32 terminal over resistor|95, filaments of the two tubes |43 and |42, resistor |96, and thenceover the contact |14 in either its No. 2 or No. 3 position to the N32terminal of the source of power. It is to be seen, therefore, that thetubes of both the frequency translating units RFI and TFT are constantlyheated as long as the cut-out switch |12 is closed, but that the powertubes of the transmitter AT are heated only when the microphone handleis moved to either the communicating or calling position.

The frequency translating unit RFT of the receiving apparatus may takeany one of several forms for such devices and as here shown includes twostages of high frequency amplification, a demodulating stage, a lter,and one stage of low frequency amplification. The specic structure ofthis device forms no part of our invention, and it will be describedonly sufciently for an understanding of our invention, it beingunderstood that volume control, stabilizing capacitances and biasingvoltages may be added as desired. The tubes |89 and |88 of the two highfrequency stages of amplification are each of the screen grid typehaving an indirectly heated cathode, the cathode in each case beingconnected with ground through a suitable biasing element. The plates ofthese tubes |89 and |88 are each connected with the BA terminal of thepower unit PU over wire 2|0, reactor |91, resistor |98 and wire |99, theconnection between the two portions of the wire |99 being indicated byarrows in order to simplify the drawings as much as possible. Theprimary winding 200 of a coupling transformer T1 interposed in theconnection of the plate of the tube 88 with the wire 2|0, and a portionof the winding 20| of a coupling transformer T8 is interposed in theconnection between the plate of the tube |89 land the wire 2|0. The gridof the first stage tube |89 is connected with the receiving coil RC in amanner to be more fully described later, and the winding 20| couplingtransformer T8 is included in the grid circuit of the second stage tube|88, and, consequently, current picked up by the receiving coil RC willbe repeated in the primary winding 200 of the transformer T1 greatlyincreased in amplitude and will induce an electromotive force ofcorresponding frequency in the secondary winding 202 of thattransformer.

In this form of our invention single side band transmission iscontemplated, a carrier of '7000 cycles per second being modulated withvoice frequencies or with the calling frequency, the carrier and oneside band suppressed and the upper side band passed. Hence, during a.telephone conversation frequencies corresponding to the upper side bandof the carrier modulated at voice frequencies are induced in thesecondary winding 202 of the transformer T1, and during a callingcondition a single frequency equal to an upper side band frequency ofthe carrier modulated at a predetermined voice frequency is induced inthe secondary winding 202. For demodulating such upper side bandfrequencies the tubes |86 and |81 of the demodulating stage areassociated with an oscillating circuit so proportioned as to generate alocal carrier of '7000 cycles, and are provided with push-pull balancedcircuits for mixing the frequencies induced in the secondary Winding 202with the local carrier. are triodes having indirectly heated cathodes,the cathodes of the tubes being connected with a ground electrode 203.The plates of the tubes |86 and |81 are connected with the BA terminalof the power `unit over a reactor 204, wire 2|0 and thence over the sameplate circuit traced for the tubes |88 and |89, the upper portion of theprimary winding 205 of a transformer T9 being interposed in theconnection between the reactor 204 and the plate of tube |86, and thelower portion of the primary winding 205 being interposed between thereactor 204 and the plate of the tube |81. The grids of the tubes |86and |81 are connected with the outside terminals of the secondarywinding 202 of the transformer T1, a center terminal of which isconnected with the cathodes over a biasing element 206 and a portion ofa reactor 201. An oscillating circuit 208 is formed to include a portionof the reactor 201 and a condenser 252, and this circuit has oneterminal connected with the center terminal of the secondary winding 202over the biasing element 208 and its other terminal connected with acenter terminal of the primary vwinding 205 of the transformer T9 over ablocking condenser 2| In other Words, the oscillating circuit 208 isconnected with the two grids through the two portions of the secondarywinding 202 of the transformer T1 and is connected with the two platesthrough the two portions of the primary winding 205 of the transformerT9. It is clear that frequencies induced in-the secondary winding 202are applied to the grids of the two tubes |86 and |81 opposite in phase,and the local carrier is applied to the two grids in phase.

As here shown these tubes i of the l coI The addition and subtraction ofthe carrier voltage and the upper side band frequency voltages when thusapplied to the grids of the tubes |06 and |81 demodulate the side bandand producein the plate circuits audio frequencies and other products ofdemodulation. That is, the upper side band frequencies are translateddown the frequency scale to audio frequencies. The audio frequencies andother products of demodulation produced in the plate circuits induceelectromotive forces of `corresponding frequencies in the secondarywinding 209 of the transformer T9, the electromotive forces induced inthe secondary winding 209 by the two portions of the primary winding 205being additive. .The secondary winding 209 is in turn connected to theinput of a band-pass filter BPF.

The band-pass filter BPF of the unit RFT shown conventionally may be anyone of several Well known types of filters and is so proportioned andadjusted as to pass the audio frequencies induced in the secondarywinding 209 and to suppress the other products of demodulation. Theoutput side of the filter BPF is connected with the grid circuit of thelow frequency amplifier tube |84, the plate ci' which is connected withthe BA terminal of the power unit over the wire |90, the primary winding2|2 of a transformer Tit being interposed between the plate and theconnection with the wire |99. The tube |04 is here shown as a pntode onecontrol grid of which is connected with the output of the bandpassfilter BPF as stated above, and 'the other control grid of which isconnected with the terminal of the primary winding 2| 2 remote from theplate over a resistor 2|3. It follows that audio frequencies applied tothe band-pass filter BPF are passed to the grid circuit of the tube |84where they are amplified and reproduced in the primary winding 2|2 ofthe transformer TIO and in turn induce an electromotive -force ofcorresponding frequencies in the secondary winding 2H of thattransformer.

'I'he secondary winding 2H supplies current to a call selecting circuit'and also) to the loudspeaker. The call selecting-circuit includes areactor 2| 9, a condenser 220 and a front contact 2|4R2 of the relay R2,the front contact 2|4 being shown in Fig. 5 remote from the winding ofthe relay for the sake of simplicity. A portion of the reactor 2|9 isconnected to the input terminals of a rectifier 22|, the outputterminals of which are connected with the relay RI, the arrangementbeing such that this call selecting circuit is tuned to respond to thecalling frequency of 700 cycles per second. The loudspeaker LS of Fig. 5is normally connected with the secondary Winding 2|1 over acircuitincluding a resistor 2I6, and hence is normally in a condition of lowsensitivity. In the event the relay R2 is released and its back contact2|5R2 is closed, which back contact is also shown remote from thewinding of the relay, the resistor 2I6 is short-circuited and theloud-speaker LS is connected with the secondary winding 2H in thecondition of full sensitivity. The relay R2 is provided with a stickcircuit which includes the back contact 222R| of relay Rl, which backcontact is shown remote from the winding of that relay for the sake ofsimplicity, and the No. 1l

position of the circuit controlling contact l5 of the unit MU. Apickupcircuit for the relay R2 is provided which includes the contact223 of a push button PB.' It is clear that with relay R2 once picked upby push 'button PB being depressed, it is then retained energized overits stick circuit, and this relayin turn closes the call selectingcircuit and switches -the `loudspeaker to a condition of lowsensitivity.- A-calling current supplied to the receiving apparatus willbe amplified, demodulated, filtered, applied to the call selectingcircuit, and the relay RI will be energized, with the result that therelay R2 will be released to disconnect the call selecting circuit andto switch the loud-speaker to a condition of full sensitivity. Inaddition to controlling the call selecting circuit and the loudspeaker,the relay R2 governs the tuning of the receiving coil RC as will now bedescribed.

'I'he receiving coil RC may be mounted at any convenient place on thecaboose or locomotive as the case may be, in inductive relation withvthe traffic rails. We have found that a linel wire paralleling thetraffic rails such as a line wire commonly employed in telegraph workand lstrung on a pole paralleling the railway, also serves as a path forthe communicating current. Consequently, the receiving coil RC may bemounted in inductive relation with a line wire if found desirable to doso, or it may consist of different portions which are mounted ininductive relation with the traflic rails and line wire, respectively.That is to say, the receiving coil RC is responsive to the communicatingcurrent flowing in the traflic rails or in the line Wire or in both. Thereceiving coil RC vis preferably of the air core type and constructed tohave a relatively low power factor.

The coil RC is normally included in a circuit sharply tuned to resonanceat the frequency of the calling current andthe tuning is broadened forreceiving a telephone current. As here shown, a normal receiving circuitcan be traced from the right-hand terminal of the coil RC over wire 224,the lower portion of a reactor 225, front contact 226 of the relay R2,condenser 221, a secondfront contact 228 of the relayvR2 and wire 229 tothe left-hand terminal of the coil RC. This circuit is so proportionedas to be tuned sharply to resonance at the frequency of the callingcurrent and hence noise currents having frequencies a few cycles to eachside of thc calling current only will be picked up duringnontransmitting periods. Upon reception of a calling current and theenergizing of the relay Rl, the relay R2 will be deenergized and thatrelay upon releasing disconnects the above traced receivirig circuit andcompletes a second receiving circuit which extends from the right-handterminal of the receiving coil RC overv wire 224, a larger portion ofthe-reactor 225, back contact 230 of relay R2, condenser 221, resistor23|, and to the left-hand terminal of the coil RC. The resistor 23| andthe other parts of this last traced circuit are so proportioned that the'tuning of the receiving circuit is'relatively broad and responds to theupper side band frequencies of the carrier having a width equal to thevoice frequency range. The reactor 225 is included in the input side of'the band-pass filter FI which filter is so proportioned and adjusted asto pass the upper side band frequencies of the voice frequency range.Consequently, the upper side band telephone current transmitted from theremote location subsequent to the sending of a calling current will bepicked up by the receiving coil RC and Apassed by the filter FI to theinput cf the frequency translating unit RFT where it will be amplified,translated down the frequency scale tion of the tube |92.

by demodulation, flltered'and then further am plified and applied to theloudfspeaker. After a telephone conversation, the receiving coil RC maybe switched to its sharply tuned circuit by the operator depressing thepush button PB and reenergizing the relay R2. l

The frequency translating unit TFI of the transmitting apparatus maytake any one of several forms and here consists of a modulatorscillator,a filter and one stage of amplification. The modulator-oscillatorincludes the pentode |93 and a conventional oscillating circuit 232. Theplate of the tube |93 is connected with the BPA terminal of the powerunit over reactor :233, wire 234, and a resistor 235. ing circuitincludes reactor 286 and the two condensers 231 and 238 in parallel, thecondenser 238 being included in the circuit over aback contact 239 ofthe calling relay CR. 'This os- :illating circuit has one terminalconnected with the plate circuit of the tube |93 over a blockingcondenser 24| and its other terminal connected with the control grid242. VThe parts of this oscillating circuit are so proportioned thatwhen ".he two condensers 231 and 238 are included in the circuit acarrier `of '1000 cycles per second is generated. The control grid 243of the tube |93 isprovided with a circuit that includes the secondarywinding 52 of the transformer T2 and in which secondary winding voicefrequencies are induced as pointed out hereinbefore. It is apparent thatthe carrier frequency will be modulated by the voice frequenciesproduced in the microphone M. The modulated carrier is applied to aband-pass filter shown conventionally over a back contact 244 of thecalling relay CR, and around which back contact a resistor 241 isconnected. Aresistor 256 may be connected across the connection of theoscillating circuit with the -filter if desired. The associated bandpassfilter BPF is so proportioned that the carrier and one side band aresuppressed and the other side band, in this instance the upper sideband, is passed. The output side of the bandpass `filter is connectedwith the grid circuit of the'amplifying tube |92, the plate of whichtube is connected with the BPA terminal of the power unit over theprimary winding 246 of a transl former TI l, wires 245 and 234, and theresistor I 235. It follows from the foregoing description of the unitTFT that with the calling relay CR deenergized and voltage applied tothe plates of the tubes |93 and |92, a carrier of 7000 cycles per secondis generated which may be modulated at voice frequencies, the carrierand the lower side band suppressed, and the upper side band increased inmagnitude by the full amplifica- In the ,event the calling relay CR ispicked up and its back contacts 239 and 244 are open, the condenser 238is discon nected from the oscillating circuit 232, and the resistor 241is interposed in the connection between that circuit and the filter. Thediscon- 'necting of the condenser v238 shifts the frequency cf thecarrier generated from 7000 to 7700 cycles which is nowapplied to thefilter, its amplitude` being attenuated by the resistor 241. I'hefrequency of '1700 cycles falling within the voice frequency range ofthe modulated carrier, it is passed by the filter and in turn isamplified by the tube |92. In other words, the voice frequenciesapplied' to the frequency translating unit of the transmitting apparatusare translated up the frequency scale to upper side band frequencies ofthe carrier, and in the event the 'I'he oscillatcalling relay is vpickedup, the' carrier is shifted by an amount equal to a given voicefrequency to produce a calling current. It will be noted that inasmuchas no potential is normally supplied at the terminal BPA of the powerunit,

the frequency translatingunit of the transmitting apparatus is normallyinactive.

The two tubes |42 and |43 of the transmitter AT are connected in apush-pull arrangement the same as described for Fig. 1.V In Fig. 5, the

grid of the tube |42 is connected with vthe sec-f.; ondary winding 248of the transformer TI i, and

the grid of the tube |43 is-ccnnected with the secondary winding 249 ofthe same transformer,

va biasing battery 250 being interposed between a common connection ofthese two secondary windings and the laments of the tubes. The plates ofthe power tubes |42 and |43 are connected .with the BP terminal of thepower unit over a wire 25|, the upper and lower portions of the primarywinding |49' of the transformer T6. being interposed inthe platecircuits of the tubes |42 and |43, respectively. The secondary winding|4| of the transformer T6 of the transmitter AT is connected with thewheels 254 and 255 of the vehicle with the result that the transmitterAT when rendered active will supply the Vupper side band frequenciesdelivered to its input circuit from the unit 'I' FT to the trafc railsamplified to a relatively high energy level. In describing the operationof the apparatus of Fig. 5, we shall assume that the cut-out switch twotranslating units are heated but only the plates of the tubes of thereceiving apparatus are supplied with voltage, since the directionalrelay DR is deenergized.4 I'he relay R2 being picked up, the callselecting circuit and the loud-speaker are connected withthe output ofthe receiving apparatus, the loud-speaker being in a condition of lowsensitivity, and the receiving coil RC is connected to the input of thereceiving apparatus sharply tuned to the calling frequency. Althoughthe. receiving apparatus is thus in a condition for receiving, noisecurrents of 'a frequency close to the frequency of the calling currentonly will be Vpicked up during the non-transmitting period and thesefrequencies will be applied to the loud-speaker in the l. lowsensitivity condition with the result that noise will be reduced to alow level. A calling current of the proper frequency when picked up bythe receiving coil RC will be passed to the frequency translating unitRFT little attenuated and will be amplified, demodulated and filteredand impressed upon the call selecting circuit for energizing the relayRI. Picking up the relay RI releases the relay R2 to transfer theloudspeaker to a condition of full sensitivity, .and to broaden thetuning of the receiving coil RC sufiiciently to pass a band offrequencies having the voice frequency range. Reception of a telephonecurrent subsequent to the receiving of a calling current will be pickedup by the coil RC and passed to the frequency translating unit RFT whereit will be amplified, demodulated, filtered, further amplified andsupplied to the loud-speaker. -'I'l'ie operator will respond to acalling current by shifting the microphone handle I4 to thecommunicating position with the result that the second dynamotor DM2will be started up and the filaments of the power tubes of thetransmitter AT will be supplied with current. To now transmit atelephone message, the operator will first depress the push button I8Ato energize the directional relay DR. Relay DR, on picking up. willdisconnect the plate circuits of the receiving tubes from the powerunit, and will supply 500 volts to the tubes of the frequencytranslating unit TFT and 1000 volts to the power tubes of thetransmitter AT. Speaking into the microphone will now cause the carrierto be modulated at the voice frequencies, the upper side band selected,this upper side band amplified to a relatively high energy level andsupplied to the traffic rails. From this point on, the operator willshift the equipment to the condition for sending or to the condition forreceiving by operating the push button to release or pick up thedirectional relay DR. When the relay is released, plate voltage isapplied to the tubes of the receiving apparatus and when it is picked upplate voltage is applied to the tubes of the transmitting apparatus. Itis to be noted, therefore, that when current is supplied to the traiiicrails, this current will be ineffective to actuate the receivingapparatus at the same location since the tubes of the receivingapparatus at that location are now without plate voltage.

Should the operator desire to send a calling current, he will shift thehandle M to the calling position which will start the second dynamotorDM2, pick up both the directional relay DR and the calling relay CR, andalso supply current to the filaments of the power tubes. The energizingof the directional relay DR will transfer the power unit from the platesof the receiving tubes to the plates of the transmitting tubes the sameas 'described above, and the energizing of the calling relay CR willcause the frequency of the carrier to be shifted from 7000 cycles to'7700 cycles which will now be amplified to a relatively high energy andsupplied to the traffic rails. Again it will be noted that the outgoingcurrent will be ineffective to actuate the receiving apparatus due tothe fact that no voltage will be applied to the plates of the tubes ofthat apparatus.

While direct and inductive couplings are provided in Fig. 5, it will beunderstood that electrostatic coupling for either transmitting orreceiving may be employed if found desirable. Furthermore, it isapparent that a single inductor coupled with traiic rails may beprovided and that inductor connected with the transmitting apparatus orwith the receiving apparatus through the medium of contacts operated bythe directional relay the same as in Fig. 1. It will be noted that thereceiving apparatus of Fig. may be transferred fromthe normal callingcondition to the telephone condition by the operator holding the handleI4 out of its normal position to move the contact l5 away from its No. lposition long enough to cause the release of the relay R2.

As stated earlier in the specification, we do not wish to limitourselves to communicating systems for railway trains, but our inventionis equally useful for other communicating systems of the modulatedcarrier current type and the application here disclosed will serve toillustrate the many places our invention may be employed.

Although we have herein shown and described only certain forms ofapparatus embodying our invention, it is understood that various changesand modifications may be made therein Within the scope of the appendedclaims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. A communicating system including; telephone equipment at each of twolocations and each comprising a sending apparatus for transmitting acarrier telephone current or a carier calling current, and a receivingapparatus for receiving such carrier current, said receiving apparatushaving a selective calling condition and a telephoning condition; acircuit extending between said locations; a controller at each locationhaving a first, a second and a third position; means effective in thefirst position of the controller to render the sending apparatusinactive and to connect the receiving apparatus to said circuit in itsselective calling condition, means effective in the second position ofsaid controller for rendering the sending apparatus active to transmit atelephone current and to selectively connect either the sending or thereceiving apparatus with the circuit, means effective in the thirdposition of the controller to connect the sending apparatus with thecircuit in an active condition to supply the calling current, andmanually operated means to govern said controller.

2. A communicating system including, transmitting apparatus at each oftwo spaced locations adapted to transmit a carrier frequency modulatedwith a signaling frequency but normally inactive, receiving apparatus ateach of said two locations adapted to receive and demodulate a modulatedcarrier frequency and normally active, a communicating circuit extendingbetween said two locations, normal circuit means at each location toconnect the receiving apparatus with said communicating circuit, callingcircuit means at each location to disconnect the receiving apparatusfrom said circuit and to render active and to connect the transmittingapparatus with said circuit, a circuit controller having a rst and asecond position, communicating circuit means at each loca.- tion torender the transmitting apparatus active and to render said circuitcontroller effective for selectively including the transmitting or thereceiving apparatus in the communicating circuit; a first manuallycontrolled means at each location to selectively govern the normal,calling and communicating circuit means; and a second manuallycontrolled means at each location to govern the circuit controller.

3. A communicating system for railway trains including, communicatingequipment comprising a transmitting apparatus and a receiving apparatus,said equipment having a normal call receiving condition, a telephonecondition and a call sending condition, a communicating circuitincluding the traic rails; a first controller having a first, a secondand a third position; a second controller having a first and a secondposition, means effective in the first position of the first controllerto establish the normal call receiving condition and to couple thereceiving apparatus to the traffic rails, means effective in the secondposition of the first controller to establish the telephone conditionand tol render effective said second controller to selectively coupleeither the transmitting apparatus or the receiving apparatus to thetrafc rails, means effective in the third position of the firstcontroller to establish the call sending condition and to couple thetransmitting apparatus to the traffic rails, a first manually operatedmeans to govern the rst controller, and a second manually operated meansto govern said second controller.

4. A communicating system for railway trains including, transmittingapparatus at each of two spaced locations on a train and adapted tosupply a carrier calling current or a carrier telephone current butnormally inactive, receiving apparatus at each of said locations adaptedto receive such carrier current and normally active, a circuit extendingbetween said two locations, a normal means at each location to couplethe receiving apparatus to said circuit, calling means at each locationto render active the transmitting apparatus to supply the carriercalling current and to couple said transmitting apparatus with thecircuit, a circuit controller having a first and a second position,telephone means at each location to render active the transmittingapparatus to supply the carrier telephone current and to render saidcircuit controller effective to selectively connect either thetransmitting apparatus or the receiving apparatus with the circuit forexchanging telephone messages between the two locations; a firstmanually operated means to selectively govern the normal, calling andtelephone means; and a second manually operated means to govern saidcircuit controller.

5. A communicating system including, a source of carrier current of apredetermined frequency, telephone means for modulating said carriercurrent with voice frequencies, manually controlled means for at timescausing said source to produce a calling current having a frequencycorresponding to said carrier modulated with a predetermined singlevoice frequency, a receiving means adapted to receive said carriermodulated with voice frequencies, a telephone receiver, operating meansgoverned by said receiving means for operating said telephone receiverbut normally ineffective, a call selecting circuit governed by saidreceiving means and responsive only to said predetermined frequency ofthe calling current, means controlled by said call selecting circuit forrendering said operating means effective to operate said telephonereceiver in response to an impulse of calling current, and means forretaining said operating means effective after the termination of saidimpulse.

6. A communicating system including, a source of calling current adaptedto supply a frequency corresponding to a predetermined carrier frequencymodulated with a predetermined single voice frequency but normallyinactive, a manually controlled means for at times rendering said sourceactive, a loud-speaker, a receiving means adapted to be influenced bysaid calling current, a call selecting circuit normally connected withsaid receiving means and responsive only to said frequency of thecalling current, an operating circuit governed by said receiving meansfor operating the loudspeaker but normally disconnected, meanscontrolled by said call selecting circuit for connecting said operatingcircuit with said receiving means in response to an impulse of callingcurrent, and means for retaining said operating circuit connected withthe receiving means after the termination of said impulse.

7. A communicating system including, a source of carrier current of apredetermined frequency, signaling means for modulating said carrierfrequency with signaling frequencies of a given range, manuallycontrolled means for at times causing said source to produce a callingcurrent having a frequency corresponding to said carrier modulated witha, predetermined single frequency within said given range, a receivingmeans adapted to receive said modulated carrier current, a signalingdevice responsive to all vfrequencies within said given range, operatingmeans governed by said receiving means for operating said signalingdevice but normally ineffective, a call selecting circuit governed bysaid receiving means and responsive only to said predetermined frequencyof the calling current, means controlled by said call selecting circuitfor rendering said operating means effective in response to an impulseof calling current, and means for retaining said operating meanseffective after the termination of said impulse.

8. A communicating system including, a source of carrier current of agiven frequency, telephone means for modulating said carrier currentwith voice frequencies, manually controlled means for at times renderingsaid telephone means ineffective and for causing said source to supply acalling current having a frequency corresponding to the carriermodulated with a predetermined frequency within the voice frequencyrange, a loud-speaker, a receiving means adapted to be influenced bysaid modulated carrier current, a call selecting circuit normallyconnected with said receiving means and responsive only to thepredetermined frequency of the calling current, an operating circuitgoverned by the receiving means for operating the loud-speaker butnormally disconnected, means controlled by the call selecting circuiteffective to connect said operating circuit with the receiving means andto disconnect the call selecting circuit in response to an impulse ofthe calling current, means for retaining the operating circuit connectedwith the receiving means at the termination of said impulse, and meansfor restoring the operating circuit and the call selecting circuit totheir normal condition.

9. A communicating system including, a source of carrier current of agiven frequency, telephone means for modulating said carrier currentwith voice frequencies, semi-automatic means for at times rendering saidtelephone means ineffective and for causing said source to supply for apredetermined interval a calling current having a frequencycorresponding to the carrier modulated with a predetermined frequencywithin the Voice frequency range, a loud-speaker, a receiving meansadapted to be influenced by said modulated carrier current, a callselecting circuit normally connected with said receiving means andresponsive only to the predetermined frequency of the calling current,an operating circuit governed by the receiving means for operating theloud-speaker but normally disconnected, means controlled by the callselecting circuit effective in response to an impulse of the callingcurrent to disconnect the call selecting circuit and to connect theoperating circuit to the receiving means whereby the loud-speaker soundsa calling note, means for retaining the operating circuit connected withthe receiving means to reproduce a telephone message at theloud-speaker, and means for restoring the operating circuit and the callselecting circuit to their normal condition.

10. A communicating system including, a communicating circuit, a sourceadapted to supply a carrier current to said circuit, an electron tubeamplifier having a plate and a grid, a transformer having one windingconnected with the plate of said tube and a second winding connectedwith said source, a microphone, means to apply voice frequenciesproduced by speaking into the microphone to the grid of Said `tube formodulating the carrier by the corresponding amplified voice frequencies,coupling means including a reactance device for at times coupling theplate to the grid of the tube for causing the electron tube to generatea given frequency for modulating the carrier with said single frequency,and manually controlled means to render either the microphone or thecoupling means active whereby either a telephone message or a callingsignal corresponding to said given frequency is delivered to saidcommunicating circuit.

11. A communicating system including, a communicating circuit, anelectron tube having a plate and two controly grids; an oscillatingcircuit including an inductor and a condenser in parallel, and connectedwith said plate and one of said grids for generating a carrier currentof a given frequency; telephone means including a microphone connectedwith the other grid of said tube for modulating `the carrier with voicefrequencies of a telephone message, calling means adapted when active toconnect a second inductor in parallel with the inductor of saidoscillating circuit to increase the carrier frequency by an amount equalto a given voice frequency, filtering means influenced by theoscillating circuit and adapted to pass to the communicating circuitonly side band frequencies of said carrier of a width equal to voicefrequencies, and manually controlled means to render either thetelephone means or the calling means effective.

12. A communicating system including, a communicating circuit, anoscillating circuit including an inductor and a condenser in parallel,means connected with said oscillating circuit to cause it to supply acarrier current of a given frequency, telephone means including amicrophone and connected with said oscillating circuit in such a manneras to modulate the carrier with voice frequencies of a telephonemessage, a second inductor, calling means for connecting said secondinductor in parallel with the inductor of the oscillating circuit toincrease the carrier frequency by an amount equal to a given voicefrequency, means to transmit the current supplied by the oscillatingcircuit to the communicating circuit, and manually controlled means torender either the telephone means or the calling means effective.

13. A communicating system including, a ccmmunicating circuit, twoelectron tubes each having a plate and a grid, an output transformerhaving its |primary Winding normally connected with said plates in apush-pull arrangement, an input transformer having its secondary windingconnected with said grids in a push-pull arrangement, a telephonecircuit connected with the primary winding of the input transformer, anoscillating circuit including an inductor and a condenser in parallel,said oscillating circuit having one ,terminal connected with said platesand the opposite terminal connected with said grids and arranged in sucha manner that a carrier current of a given frequency is generated andmodulated with the voice frequencies produced in the telephone circuitvand then is balanced out leaving only the side band frequencies in thesecondary winding` of the output transformer; calling means to vary theoscillating circuit to shift the frequency of the carrier from saidgiven frequency by an amount corresponding to a predetermined voicefrequency, means governed by the calling means to transfer the plates ofsaid tubes to a parallel arrangement to deliver to the outputtransformer the shifted carrier, means to couple the secondary of theoutput transformer to the communicating circuit, and manually controlledmeans to render either the telephone circuit or the calling meansactive.

14. A communicating system including, a cornmunicating circuit, twoelectron tubes each having a plate and a grid, an output transformerhaving its primary winding normally connected with said plates in apush-pull arrangement, an input transformer having its secondary windingconnected with said grids in a push-pull ar rangement, a telephonecircuit connected with the primary winding of the input transformer, anoscillating circuit including an inductor and a condenser in parallel,said oscillating circuit having one terminal connected with said platesand the opposite terminal connected with said grids and arranged in sucha manner that a carrier current of a given frequency is generated andmodulated with the voice frequencies produced in the telephone circuitand then is balanced out leaving only the side band frequencies in thesecondary winding of the output transformer; calling means to decreasethe capacity of the condenser of said oscillating circuit to increasethe carrier frequency by an amount corresponding to a predeterminedvoice frequency, means governed by the calling means to transfer theplates of said tubes to a parallel arrangement to deliver to the outputtransformer the shifted carrier corresponding to predetermined upperside band frequency, means to couple thel secondary of the outputtransformer to the communicating circuit, and manually controlled meansto render effective either the telephone circuit or the calling means.

15. A communicating system including, a communicating circuit, telephonemeans adapted to supply frequencies corresponding to a given carrierfrequency modulated with the voice frequencies of a telephone message tosaid communicating circuit, calling means adapted to supply a singlefrequency corresponding to said carrier frequency modulated with apredetermined voice frequency to said communicating circuit, a manuallyoperated means having a communicating position for rendering thetelephone means active and a calling position'for rendering the callingmeans active, and automatic means effective in response to the callingposition of said manually operated means fo1` maintaining the callingmeans active for a given time interval regardless of the subsequentposition of said manually operated means.

16. In combination, a receiving circuit adapted to receive currenthaving a frequency range corresponding to the voice frequencies, aloudspeaker, means controlled by the receiving circuit for operating theloud-speaker, a control device for establishing the loud-speaker in acondition of low sensitivity or in a condition of full sensitivity andnormally at the condition of low sensitivity, a call selecting meansgoverned by the receiving circuit and including a tuned circuit networkresponsive to a given single frequency only within the voice frequencyrange, means controlled by the call selecting means for causing thecontrol device to establish the loudspeaker in a condition of fullsensitivity in response to an impulse of current of said givenfrequency, and holding means for retaining the loud-speaker in thecondition of full sensitivity.

i7. In combination, a receiving circuit adapted to receive frequenciescorresponding to a given carrier frequency modulated with voicefrequencies, a demodulator having its input influenced by the receivingcircuit, a loud-speaker, operating means controlled by the output of thedemodulator for operating the loud-speaker, a `elay for governing theoperating means and normally deenergized to establish the loudspeaker ina condition of low sensitivity, an amplifier governed by thedemodulator, a control circuit governed by the amplifier and selectivelyresponsive to a given frequency only within the voice frequency range,means controlled by said control circuit to energize said relay inresponse to an impulse of current of said given frequency to establishthe loud-speaker in a condition of full sensitivity, a holding circuitincluding a local source of current for retaining said relay energized,and manually controlled means for governing said holding circuit.

18. In combination, a receiving circuit adapted to receive side bandfrequencies of a carrier frequency modulated with voice frequencies, ademodulator influenced by the receiving circuit, a loud-speaker governedby the demodulator and having a low sensitive condition and a fullsensitive condition, means for normally establishing the low sensitivecondition of the loud-speaker, a call selecting circuit governed by thedemodulator and including a device tuned to a given voice frequency,means controlled by said device for establishing the full sensitivecondition of the loud-speaker in response to an impulse of current ofsaid given frequency, holding means for retaining the loud-speaker inthe full sensitive condition, and manually controlled means forgoverning the holding means.

19. In combination, a receiving circuit adapted to receive frequenciescorresponding to a given carrier frequency modulated with voicefrequencies, a demodulator influenced by the receiving circuit, aloud-speaker, an operating circuit for connecting the loud-speaker withthe demodulator but normally disconnected, a tuned device responsive toa given voice frequency only, a call selecting circuit for connectingsaid device with the demodulator and normally connected, meanscontrolled by said device in response to an impulse of current of saidgiven frequency to disconnect said call selecting circuit and to connectthe operating circuit with the demodulator, and manually controlledmeans to restore said operating circuit and said call selecting circuitto their normal condition.

20. A communicating system for railway trains including a communicatingcircuit comprising the traflic rails, a train-carried receiving circuitinductively coupled with the traf-llc rails and adapted to be influencedby a carrier current lmodulated with voice frequencies, a demodulatingmeans having its input influenced by said receiving circuit, a callselecting circuit controlled by the output `of said demodulating meansand selectively responsive to a given frequency within the voicefrequency range, a loud-speaker, an operating means controlled by theoutput of said demodulating means for operating the loud-speaker butnormally inactive,

`and means controlled by the call selecting circuit for automaticallyrendering said operating means active in response to an impulse ofcarrier current modulated at said given frequency picked up from thetrailic rails.

21. A communicating system including, a communicating circuit, a sourceof calling current adapted to supply a frequency corresponding to a sideband frequency of a given carrier modulated with a predetermined voicefrequency to the communicating circuit but normally inactive, manuallycontrolled means for at times rendering said source active, receivingmeans coupled to the communicating circuit and adapted to receive anddemodulate said calling current, a loud-speaker, an operating circuitgoverned by the receiving means for operating the loudspeaker butnormally disconnected, control means selectively responsive to saidpredetermined voice frequency, a call selecting circuit governed by thereceiving meansfor energizing the control means and normally connected,means effective when said control means is energized to disconnect thecall selecting circuit and to connect the operating circuit to thereceiving means whereby the loud-speaker is switched to the receivingmeans and a call note sounded in response to the calling current.

22. A communicating system including, a communicating circuit, a currentgenerating means adapted to supply to said circuit a frequencycorresponding to a given carrier frequency modulated with apredetermined voice frequency but normally inactive, receiving meanscoupled to the communicating circuit and adapted to receive anddemodulate the current from said source, a loud-speaker governed by saidreceiving means but normally disconnected, a selecting circuit normallyconnected with said receiving means and selectively responsive to saidpredetermined frequency only, means controlled by said selecting circuitto connect said loud-speaker with the receiving means in response to animpulse of current of the predetermined frequency, and manually operatedmeans for rendering said current generating means active in accordancewith a prearranged code to establish a corresponding code of currentimpulses whereby said loudspeaker is automatically rendered active andnotes corresponding to said code are sounded.

23. A communicating system for railway trains including, a communicatingcircuit comprising the traflic rails, a transmitting circuit at onelocation n a train and inductively coupled with the traffic rails, atransmitting means for supplying to the transmitting circuit a frequencycorresponding to a given carrier modulated with a predetermined voicefrequency but normally inactive, a receiving circuit located at a secondpoint on the train and inductively coupled with the` trailic rails,receiving means controlled by said receiving circuit and adapted todemodulate the current from said transmitting means, a loudspeakergoverned by said receiving means, and manually operated means forrendering the signaling means active in accordance with a prearrangedcode to establish a corresponding code of current impulses whereby saidloud-speaker is rendered active to sound notes corresponding to saidcode.

24. A communicating system including; telephone equipment at each of twolocations each of which comprises a. sending apparatus capable ofsupplying either a carrier telephone current or a carrier callingcurrent but normally inactive, and a receiving apparatus capable ofreceiving and demodulating such carrier currents and normally active; acalling, a transmitting and a directional relay at each location; acommunicating circuit extending between said two locations, meanscontrolled jointly by said transmitting and directional 4relays forrendering the sending apparatus active, means controlled by said callingrelay for rendering the sending apparatus effective to supply a callingcurrent, means controlled by said directional relay for selectivelycoupling either the sending or the receiving apparatus with saidcircuit, and manually operated means for selectively governing saidrelays.

25. A communicating system including, a communicating circuit, anelectron tube provided with a plate and two control grids, anoscillating circuit including an inductance and a capacitance connectedwith said plate and one of said grids for generating a carrier currentof a predetermined frequency, telephone means including a microphoneconnected with the other of said grids for modulating the carrier withvoice frequencies, transmitting means coupled with said oscillatingcircuit for supplying current to the communicating circuit and includinga filter operative to pass a given side band of said carrier of a widthequal to the voice frequencies, calling means connected with saidoscillating crcuit including a reactance device and a circuitcontrolling contact and. operative to vary the frequency of the carriercurrent by an amount equal to a given voice frequency, and manuallycontrolled means for operating said circuit controlling contact.

26. In combination, a receiving circuit including a lter operative topass a carrier current modulated with voice frequencies, a demodulatorhaving its input influenced by the receiving circuit, a loud-speaker, acontrol relay, an operating circuit controlled by the output of thedemodulator for operating the loud-speaker and including a back contactof said relay, a call selecting circuit governed by the output of thedemodulator including a tuned device responsive to a given voicefrequency only, a pickup circuit for said control relay including anormally open manually operated contact, and a stick circuit for saidcontrol relay including a Contact operated by the tuned device and anormally closed manually operated contact, whereby the loudspeaker isretained normally inactive and is established in an active conditioneither automatically upon reception of a calling current of said givenfrequency or manually by operation of said normally closed contact.

27. A communicating system comprising, a communicating circuit extendingfrom a location, transmitting means at the location including anelectron amplifying tube and a microphone connected with the grid of thetube, receiving means at the location including an electron amplifyingtubeand a loud-speaker connected with the plate of the tube, a currentsource, a directional relay, means for connecting said current sourcewith the plate of the receiving amplifying tube including a back contactof said relay, means for connecting said current source with the plateof the transmitting amplifying tube including a front contact of saidrelay, control means for governing said relay including a manuallyoperated circuit controller, and means for coupling the output of thetransmitting means and the input of the receiving means with saidcommunicating circuit.

28. A communicating system comprising; transmitting means including amicrophone, an electron tube oscillator, a lter and an electron tubeamplier; receiving means including an electron tube amplifier, anelectron tube demodulator, a filter anda loud-speaker; a. circuitsource, a directional relay, means for connecting said. current sourcewith the plates of the tubes ci the receiving means including a backcontact of the directional relay, means for connecting said currentsource with the plates of the tubes or" said transmitting meansincluding a front contact of the directional relay, and circuit meansfor controlling the directional relay including a manually operatedcircuit controlling contact.

29. A communicating system comprising; transmitting means including amicrophone, an electron tube oscillator, a filter, an electron tubeamplier and a sending circuit; receiving means including a receivingcoil, an electron tube amplifier, an electron tube demodulator, a lterand a loud-speaker; a current source, a directional relay, means forconnecting the source with the plates of the tubes of the receivingmeans for reproducing the telephone message of a carrier side bandtelephone current picked up by the receiving coil and said meansincluding a back contact of said relay, means for connecting the so-urcewith the plates of the tubes of the transmitting means for supplying tothe sending circuit a carrier side band telephone current and said meansincluding a front contact of said relay, and circuit means forcontrolling said relay including a manually operated circuit controllingcontact. 30. A communicating system comprising; transmitting meansincluding a microphone, an electron tube oscillator, means formodulating the output of said oscillator with voice frequencies producedin the microphone and a sending circuit; receiving means including areceiving coil, an electron tube demodulator and a loudspeaker; a-current source, means for connecting the current source with the plateof said demodulating tube for reproducing the telephone message of acarrier telephone current picked -up by the receiving coil and includinga. rst circuit controlling contact, means for connecting the currentsource with the plate of said oscillating tube for transmitting over thesending circuit a carrier telephone current and including a secondcircuit controlling contact, and manually operated means for selectivelyoperating said contacts.

31. A communicating system for railway trains comprising, acommunicating circuit including the traine rails, train-carriedreceiving means including anelectron tube demodulator, a loudspeaker anda conductor electrically coupled with the traffic rails; transmittingmeans includingia microphone, an electron tube oscillator and aconductor electrically coupled with the traffic rails; a current source,means for connecting the current source with the plate of thedemodulating tube for reproducing at the loudspeaker a carrier telephonecurrent picked up by the conductor of the receiving means and said meansincluding a circuit controlling contact, other means for connecting thecurrent source with the plate of the

